Professional sports teams, because of the close contact amongst players, are always ripe for outbreaks of contagious diseases. In the recent past MRSA has been one organism associated with sports-related outbreaks (see the St. Louis Rams). Today, the NHL is now facing an outbreak of mumps that has plagued several teams.

Mumps is a viral disease and is generally preventable through vaccination. It is one of the "M's" in the MMR vaccination. This vaccine, like many others, is due to the genius of Maurice Hilleman--one of the true heroes of infectious disease. It is known for causing characteristic swelling of the salivary glands but can also meningitis or orchitis (inflammation of the testicles).

However, over the past several years we've seen large outbreaks of mumps occur in certain population that are somewhat dense such as amongst college students. Interestingly, these outbreaks are occurring in relatively highly vaccinated groups of individuals (vs. measles, for example).

So why are these outbreaks occurring? When a vaccine fails I think of one of two causes: the pathogen has mutated or the immunity induced by the vaccine has fallen below a protective threshold. While the strains causing these outbreaks (G) is different than the vaccine strain (A), G strain outbreaks are halted by the A strain vaccine. So strain disparity doesn't seem to be the explanation. There has been some suggestion that a third booster dose of the vaccine can bring outbreaks to a halt supporting the waning immunity theory.

Another factor is something called "exposure pressure." Since these outbreaks are occurring in specific "closed" populations could it be that the amount of exposure an adequately vaccinated individual has to the virus in these outbreak settings overwhelms the immune system?

My hypothesis is that it is probably waning immunity coupled to exposure pressure that explains these outbreaks. Outside of outbreak settings mumps has become much less common and any natural boosting of vaccine immunity from exposure to natural cases is no longer present, allowing antibody levels to fall. Those with waning immunity are simply unable to resist the virus when faced with the exposure pressure in a closed population in which mumps is being harbored.

There are reports that some teams are moving forward with booster doses, which should stem the outbreak. But, it remains important to understand more fully why these outbreaks continue to occur and whether universal 3rd boosters should be considered for certain populations.